"Cryopreservation" is a technique for freezing and storing cellular and tissue matter such as blood vessels, which include veins and arteries, at extremely low temperatures while preserving the viability and function of the tissue. Each year, 360,000 small vessel coronary bypass "jumps" are performed in the U.S. alone. Another 100,000 peripheral vascular procedures, below the umbilicus, are also performed. Of the small vessel procedures, 15% are performed on patients who have already had a previous operation resulting in a lack of suitable available tissue or on patients who are diabetic or have a disease which renders the tissue less than adequate. Clinically, the only alternative is to use less than optimal tissue or use artificial vessels which are prone to occlusion and thus are less than ideal. Because of the successes resulting from the cryopreservation of heart valve tissue (see copending application Ser. No. 000,095 filed Jan. 2, 1987, which is incorporated by reference in its entirety herein), and to date, more than 3,000 cryopreserved valves and approximately 2,200 implants, it is the intention to expand this technology to vein and artery tissue as well. Thus, in the clinical setting, cryopreserved tissue would fill a need for the aforementioned patents and would in addition lead to less trauma for the patient and reduce surgical time and expense.
Previous attempts at the use of allograft vessels have met with a variety of problems. The primary concern was inconsistency in the method of harvest coupled with an inability to freeze and store the tissue properly until its intended use. In addition, previous investigators failed to perform the freezing process using state of the art techniques, and consequently, the viability of the tissue was low and inconsistent and resulted in early loss of patency.
Although there have been a few published reports on the cryogenic preservation of veins and arteries, there has been no published systematic examination for the cryobiological variables involved in the preservation procedure. Most investigators have simply infiltrated the vessel with dimethyl sulfoxide (DMSO) and rapidly frozen the tissue in liquid nitrogen. Several other investigators have used uncontrolled and unmeasured freezing rates. When dissected from the body, blood vessel tissue has a natural tendency to constrict. Investigations to date show that under such conditions the endothelial lining of the vessel may be denuded; therefore, if such a vessel is transplanted, it may be prone to thrombosis.
Preservation of the endothelial lining of these vessels is of particular importance, because the internal endothelial lining of the blood vessels actively inhibit thrombosis. Previous studies of saphenous vein cryopreservation indicate that the major abnormality in the frozen and thawed tissue was destruction and loss of this tissue layer. A primary goal of cryopreservation of the tissue is the prevention of ice crystals which damage or destroy cellular structure. Different freezing methods are applicable to particular tissues; not all tissues are alike in their ability to withstand cryopreservation and thawing yet maintain effective viability. No investigator is known to have successfully applied this technology to the internal mammary artery or other arterial tissue.